Generally, in solid state image sensors such as a CCD or the like, there has been considered a method whereby an overflow drain is provided in the photosensing surface to prevent the blooming or the overflow carriers are extinguished using the surface recombination.
In particular, the latter method is known by, for example, United Kingdom Patent Publication Gazette GB No. 2,069,759A (applicant, N. V. Philips; inventors, Marnix Guillaume Collet et al), and the like. Such a method has advantages such that sensitivity is high since an aperture in the photosensing surface is not sacrificed and that horizontal resolution is raised since the integration degree can be improved, and the like.
FIGS. 1-3 show diagrams to describe such a method of preventing the blooming by the surface recombination, in which FIG. 1 shows a front view of an ordinary frame transfer type CCD.
In the drawings, a reference numeral 1 denotes a photosensing part consisting of a plurality of vertical transfer registers having photosensitivity.
On the other hand, a numeral 2 indicates a storage part consisting of a plurality of vertical transfer registers which are shielded against the light. 3 represents a horizontal transfer register which simultaneously shifts the information in the respective vertical transfer registers of the storage part 2 by one bit, thereby taking them in this horizontal transfer register. The register 3 then performs the horizontal transfer operation so that a video signal can be obtained from an output amplifier 4.
Generally, the information formed in each vertical transfer register of the photosensing part 1 is vertically transferred to the storage part 2 in the vertical blanking interval in the standard television system and is sequentially read out on a line by line basis by the horizontal transfer register 3 in the next vertical scanning interval.
The photosensing part 1, storage part 2 and horizontal transfer register 3 are respectively two-phase driven and their respective transfer electrodes are indicated by P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5, and P.sub.6 and the transfer clocks are represented by .phi..sub.P1, .phi..sub.P2, .phi..sub.P3, .phi..sub.P4, .phi..sub.P5 and .phi..sub.P6 respectively.
FIG. 2 is a diagram showing a potential profile under such transfer electrodes P.sub.1 -P.sub.6. Low-potential portions and high-potential portions are formed under the respective electrodes provided on, for example, a p-type silicon substrate 6 through an insulating layer 5 by way of ion implantation or the like. For example, when a low-level voltage -V.sub.1 is applied to the electrodes P.sub.2, P.sub.4 and P.sub.6 and a high-level voltage V.sub.2 is applied to the electrodes P.sub.1, P.sub.3 and P.sub.5, the potentials such as indicated by the solid lines in FIG. 2 are formed. On the other hand, when the low-level voltage -V.sub.1 is supplied to the electrodes P.sub.1, P.sub.3 and P.sub.5 and the high-level voltage V.sub.2 is supplied to the electrodes P.sub.2, P.sub.4 and P.sub.6, the potentials such as indicated by the broken lines in FIG. 2 are formed.
Therefore, by applying the alternating voltages having opposite phases to each other to the electrodes P.sub.1, P.sub.3, P.sub.5 and to the electrodes P.sub.2, P.sub.4, P.sub.6 the carriers are sequentially transferred in one direction (to the right in the drawing).
In addition, the alternate long and short dash lines in FIG. 2 show the potentials when a large positive voltage V.sub.3 is applied to the electrodes. Since the wells of these potentials are in the inverting state, the overflow carrier of not smaller than a predetermined amount will have been recombined with the majority carrier and will be extinguished.
FIG. 3 is a diagram showing such a relation between the electrode voltage and the shape of the interval potential with respect to the direction of thickness of the semiconductor substrate 6. It can be seen from FIG. 3 that the potential well for the electrode voltage V.sub.3 is shallow, so that the overflow carrier is in a second state in that it is recombined with the majority carrier at the interface with the insulating layer.
On the other hand, the potential state becomes the accumulation state as a first state at the electrode voltage -V.sub.1, so that the majority carrier is easily collected around the interface; for example, this majority carrier is supplied from a channel stopper region (not shown).
Therefore, by alternately applying the voltages -V.sub.1 and V.sub.3 to the electrode P.sub.1 in the state in that a carrier is formed by, for example, applying the voltage -V.sub.1 to the electrode P.sub.2, the minority carrier to be accumulated under the electrode P.sub.1 is limited to not larger than a predetermined amount.
However on the contrary, to effectively extinguish the overflow carrier, the accumulation state and the inverting state have to be alternately formed at a high speed in the semiconductor substrate in the accumulating interval; therefore, this causes a problem such that electric power consumption is large. In addition, if such a pulse control is performed at high speed, there will be also caused a problem such that the noise to be caused by this pulse is mixed with the signal. Also, there is a problem such that the dark current drift may easily occur due to such a pulse.